[Infra] Clean up proxy-db matrix job display names

Default GHA matrix job names join every matrix field, producing unreadable
check labels like:
  'proxy-db (logging-misc, tests/proxy_unit_tests/test_proxy_reject_logging.py
   tests/proxy_unit_tests/test_audit_logs_proxy.py ..., 8, loadscope, "", 15)'

Set the job's display name to '${{ matrix.test-group }}' so each check
shows just 'logging-misc', 'proxy-utils-a-h', etc.

.github/workflows/test-unit-proxy-db.yml

@@ -74,6 +74,10 @@ jobs:
 
   proxy-db:
     needs: assert-shard-coverage
+    # Display only the semantic shard name in the checks UI instead of GHA's
+    # default "proxy-db (key-generation, tests/proxy_unit_tests/…, 0, loadscope, "", 20)"
+    # which includes every matrix field and gets truncated past the test-path.
+    name: ${{ matrix.test-group }}
     permissions:
       contents: read
       id-token: write